Fluid film bearings have found many useful places in industry for their nearly frictionless, zero wear and high accuracy characteristics. These bearings utilize a pressurized gas or liquid to separate two surfaces and provide relative motion. Due to the low viscosity of the fluids employed, the shear forces required to allow motion remain very small. In cases where a gas becomes the operating fluid, the shear forces, especially for static friction, can become nearly immeasurable. This minimal friction translates into practically no wear or surface erosion of the mating parts. In addition, this non-contacting nature averages local irregularities of the mating surfaces, hence improving straightness or roundness of motion. This is a great benefit to devices that require motion with high repeatability and accuracy such as semiconductor wafer scanning tools, coordinate measuring machines or high-speed pick and place machinery.
In order for this type of bearing to function properly, the gap separating the two or more elements needs to be very small to avoid instability of the fluid film. Most applications of fluid bearings have separation gaps ranging from less than one micron to 40 microns; with the average lying in the 5-10 micron range. This makes proper installation cumbersome, especially in the production environment or out in the field, due to the difficulty in measuring such small distances. To provide installation, some machine designers will involve the use of an adjustment screw or flexure to set the preload of the bearing. After the bearing is placed into the assembly, a threaded ball stem applies the load in the center of the bearing. The technician then adjusts the screw until the proper gap is measured. Even with fine pitch screws, it can be difficult and tedious to make adjustments in the micron range, re-measure the gap and then readjust the screw. In addition, often times it is difficult to access the bearing through the surrounding machine structure to perform a gap measurement. However, when access is available, this is typically performed by situating a suitable measuring indicator or non-contact sensor in such a way to measure movement of the bearing housing and then, in the case of an externally pressurized bearing, cycling the fluid pressure on and off while recording the relative distance the housing has translated. This method, used extensively, has a number of drawbacks related to the aforementioned access difficulty.
First, it is a measurement relatively far away from the actual film gap. The measured displacement of the bearing housing may not accurately reflect the actual width of the fluid film. Second, often measurements are only performed near the periphery of the housing and, hence, do not take into account the possibility that the bearing is lifting in a tilted state. Third, bench-top measurements often performed prior to installation are under ideal loading conditions and may not accurately reflect fluid gaps after installation. Fourth, in the field, the technician often has a reduced number of measuring capabilities at his or her disposal and may often rely on only experience or more indirect, correlated or interpolated measurements of the gap. Therefore, correct installation of the bearing is often more dependent on proper design and function of the bearing rather than on an accurate measurement of the gap.
In addition, machine tools incorporated with fluid film bearings often do not monitor the gap during operation due to the all the aforementioned technical difficulties associated with performing gap measurements. Under certain circumstances, the machine assembly may have become unknowingly stressed which may cause slight misalignment of critical features relating to the proper fluid gap. This would result in subpar performance of the machine. At this point, the technician would need to halt use of the machine, disassemble and then verify gaps of all the bearings; a costly and time consuming process.
Therefore, there exists a need to perform fluid film gap measurements more easily and accurately.
U.S. Pat. No. 6,925,854 to Neumann describes a method of verifying and inspecting the bearing gap of a hydrodynamic bearing. A measuring fluid is caused to flow through the bearing gap. This takes place prior to final assembly, and before filling lubricant into the bearing, so fluid can still flow through the bearing gap. The flow needs to be measured parallel to the longitudinal axis of the shaft and the test bearing must be open at both ends to allow the flow to be established.
Measurement of various aspects of a fluid bearing after it is already installed and operating are known. For instance, U.S. Pat. No. 7,744,281 to Fuerst et al. describes a method of monitoring temperature in a plain bearing working with a lubricating film. The method involves extracting samples of the fluid for temperature measurement. The method has several disadvantages. First, the temperature must be measured directly after extraction. Further, the amount of fluid extracted must balance the need to extract a small amount so as not to disturb the film pressure with the need extract enough fluid to keep temperature change small. Placement of a temperature sensor is also limited to a distance in which the fluid temperature will not be distorted. The method is further contingent on fluid flow of extracted samples.
Control of operating bearings through sensed pressure is also known. For example, U.S. Pat. No. 4,569,562 to Sato et al. discloses a method for powering off or switching pressure supplied to an operating bearing when pressure reaches a predetermined value exceeding a servo-controlled upper or lower limit. In this manner, the sensed data is used to dynamically hold the fluid (air) pressure within a safe range between upper and lower limits.
U.S. Pat. Nos. 5,364,190 and 5,447,375 to Ochiai et al. also teach the use of sensed temperature and pressure of fluid (oil) to constantly maintain the fluid gap of a hydrostatic bearing apparatus. The pressure and temperature of the oil are measured and used to adjust the flow rate and temperature of oil delivered to a static pressure pocket. Similarly, U.S. Pat. No. 6,547,438 to Shima discloses use of sensed temperature and pressure of a lubricant oil to control flow of the oil to the fluid gap of a hydraulic bearing device and control separation between the bearing components.